In general, silicone prostheses are inserted into the human body for various purposes such as plastic surgery, and the like. As a representative example, artificial breast prostheses are used in reconstructive surgery when breast loss occurs due to diseases or accidents and in cosmetic surgery to treat a malformed breast. In terms of anatomy, artificial breast prostheses are also used for the substitution of organs or tissues.
Artificial breast prostheses are products in which a sufficient amount of a filling material, such as saline, hydro-gel, and silicone gel, is filled in an envelope formed of silicone that can be used to manufacture artificial organs (hereinafter, referred to as a “shell”), which are devices for substitution of organs in the body. These artificial breast prostheses may be classified into products according to filling materials contained therein, may be classified into round type products and anatomical type products, which are of water droplet type, according to the shape of a product, and may be classified into smooth products and textured products according to surface condition.
For example, a saline filled artificial breast prosthesis is configured such that saline is injected or is injectable into a shell formed of silicone (e.g., polydimethylsiloxane (PDMS), polydiphenylsiloxane, and polyorganosiloxane). The saline filled artificial breast prosthesis has a structure consisting of a silicone shell and a valve.
Although the saline filled artificial breast prosthesis ensures user safety even if the filling material leaks into the human body after rupture of the shell as a result of using sterile saline as the filling material, and is easy to change the volume of a breast by adjusting the injection amount of saline, the saline filled artificial breast prosthesis is significantly deteriorated to the touch after surgery as compared to other artificial breast prostheses and the shell thereof has inferior durability.
A hydro-gel filled artificial breast prosthesis is configured such that hydro-gel composed of monosaccharide and polysaccharides is filled within the shell as in the above-described saline filled artificial breast prosthesis. The hydro-gel filled artificial breast prosthesis was developed based on the principle that the filling material can be absorbed into and excreted from the human body even if the filling material leaks due to rupture of the prosthesis.
However, in the case of the hydro-gel filled artificial breast prosthesis, long-term safety has not been established, volume change over time and occurrence of wrinkles may increase after the artificial breast prosthesis is implanted, and feeling is unnatural as compared to a silicone artificial breast prosthesis. Accordingly, the hydro-gel filled artificial breast prosthesis is not currently distributed in the market as safety thereof has yet to be proven.
A silicone gel filled artificial breast prosthesis is configured such that a shell is filled with a silicone gel having an appropriate viscosity. The silicone gel filled artificial breast prosthesis has superior product durability and a more pleasant texture than the saline filled artificial breast prosthesis and thus achieves a dominant position in the market. Although the Food and Drug Administration of the United States of America (FDA) has imposed limitations on use of silicone gel filled artificial breast prostheses due to safety issues, the use of silicone gel filled artificial breast prostheses was again allowed officially in 2006.
The silicone gel filled artificial breast prosthesis has been developed in the order of a first generation prosthesis, a second generation prosthesis, and a third generation prosthesis. This development history will be described in detail as follows.
The first generation silicone gel filled artificial breast prosthesis is a product sold from the middle of the 1960s to the middle of the 1970s, and was initially developed in 1961 by Cronin and Gerow. The first generation silicone gel filled artificial breast prosthesis can be represented in brief by the use of a thick shell, a smooth surface, and a high viscosity silicone gel. This prosthesis suffers from gel bleed and capsular contracture, but a rupture speed thereof is relatively low due to the use of the thick shell.
The second generation silicone gel filled artificial breast prosthesis is a product sold from the middle of the 1970s to the middle of the 1980s, and includes a thin shell and a silicone gel filling material of a low viscosity, for the sake of smoother texture. This prosthesis is characterized by a similar gel bleed rate, higher rupture occurrence, and lower capsular contracture as compared to the first generation prosthesis.
The third generation silicone gel filled artificial breast prosthesis is a product sold from the middle of the 1980s to the present, and includes a gel bleed barrier layer to prevent gel bleed. The third generation silicone gel filled artificial breast prosthesis includes a thicker shell and silicone gel of a higher viscosity as compared to the second generation prosthesis. In addition, a product having a rough surface has been developed, in order to reduce capsular contracture.
The above-described artificial breast prostheses commonly include a shell 1, a filling material 2, and a patch bonding portion (hereinafter, referred to as “patch portion 6”).
The shell 1 constituting a conventional artificial breast prosthesis is generally prepared using a dipping method or a spray method. When the shell 1 is prepared by dipping or spraying, silicone liquid is continuously flowed downward due to gravity in a drying process after dipping a mold in a silicone solution or spraying a silicone solution to a mold, and thus, the shell 1 obtained after the drying process has a thickness that increases toward a lower portion thereof as illustrated in FIG. 2. In particular, the thickness of a perimeter region of the shell 1 (generally referred to as “radius” in the art, a perimeter region having a curved surface) is very small as compared to a total thickness of the shell 1 and very large difference between the thickness of the perimeter region thereof and the thickness of upper and lower parts of the shell 1 occurs.
Such difference in thickness of each of a plurality of portions of the shell 1 causes differences in physical properties and stress (i.e., shear stress, normal stress, and torsional stress). Due to the differences in the physical properties and stress, different portions of the shell 1 undergo different degrees of elastic elongation with respect to given external pressure, and thus an elastic body such as a shell has a portion relatively vulnerable to high pressure and repeated fatigue.
Thus, artificial breast prostheses having such part relatively vulnerable to stress, i.e., a stress concentrated part 7 have limited durability and reduced lifespan due to fatigue, consequently leading to rupture of the artificial breast prostheses.
To address the problem of reduced durability of the shell, various technologies have been disclosed. U.S. Pat. No. 6,605,116B2 discloses a prosthesis configured such that the average thickness of the shell in the perimeter region is greater than the average thickness of the shell in the other regions in order to address the reduced durability of the shell occurring due to a thin perimeter portion of the shell, and a manufacturing method thereof.
US 2011/0046729A1 also discloses a prosthesis configured such that the average thickness of the shell in the perimeter region is greater than the average thickness of the shell in the other regions in order to address the reduced durability of the shell occurring due to a thin perimeter portion of the shell, and a manufacturing method thereof.
In the prostheses according to the related arts, although the thickness of the shell in the perimeter region is larger than the thickness of the shell in the other region and, accordingly, the perimeter region of the shell has stronger physical properties than the other region of the shell, there are still problems of differences in the thicknesses and physical properties of the regions of the shell. That is, the reinforced perimeter region of the shell causes differences in the thickness and physical properties of the adjacent parts of the shell, and thus the shell still has a stress concentration part, which leads to reduction in durability.
In addition, US 2010/0178414A1 discloses a method of manufacturing a shell by rotating a mold coated with a silicone solution during a drying process of the silicone solution and a device manufactured using the method.
More particularly, during the drying process of the silicone solution, the mold coated with the silicone solution is continuously rotated around an axis tilted at a certain angle with respect to a level surface, thereby changing a flow direction of the silicone solution and controlling movement of the silicone solution. Accordingly, the silicone solution is uniformly dispersed thereon and a shell having a uniform thickness is obtained.
The manufacturing method of the shell using the rotation method can reduce a difference in thicknesses of upper and lower parts of the shell 1 and a difference in thickness of the perimeter region of the shell 1 as compared to the manufacturing method of the shell without using the rotation method. As illustrated in FIG. 3, however, it is impossible to completely eliminate a difference in the thicknesses of all the parts of the shell 1.
From a rheological point of view, the above-described manufacturing method does not take into consideration a three-dimensional structure of a mold having different slopes in regions thereof. From the rheological point of view, a difference in flow rate of the silicone solution in each region of the mold occurs according to the three-dimensional structure of the mold having slope variation in each region thereof. Thus, the thickness of an upper part of the shell is larger than that of a lower part of the shell and thickness difference in each region of the shell occurs according to the three-dimensional structure of the mold.
In addition, as to rotational velocity and the viscosity of the silicone solution, the mold is continuously rotated around a rotation axis, and thus, difference in flow rate of the silicone solution in each region of the mold may occur according to a separation distance of the silicone solution from the rotation axis, whereby the thickness of the shell may increase based on the rotation axis.
Such thickness difference further increases due to increased surface area of the mold used as the size of products increases.
Actually, a round type shell 1 manufactured using the conventional rotation method is configured as illustrated in FIG. 3 such that the thickness of an upper part of the shell 1 is larger than that of a lower part of the shell 1. In addition, as illustrated in FIG. 3(b), an anatomical type shell 1 is also formed as the round type prosthesis 1 such that the thickness of an upper part thereof is greater than that of a lower part thereof. In addition, due to a three-dimensional structure of a mold in which left and right sides of the mold are asymmetrical, thickness difference occurs such that the thickness of one side of an upper inclined plane of the mold is generally very small, while the thickness of the other side thereof is very large, and thus the stress concentration part 7 relatively vulnerable to stress is still formed on the shell 1. In addition, conventional artificial breast prostheses have been used with existing problems being unsolved, i.e., high risk of fatigue fracture due to limitation on durability when used.